Chorde

Bugfixes
- Fix reraising of tornado.web.HTTPError from inside async
futures. Tornado exceptions don't behave as other exceptions
and need special treatment

Bugfixes
- Fix an instance of overallocation that could turn into a DoS
where we could be allocating arbitrarily large byte arrays.
This was affecting mostly the coherence protocol in some
exceptional circumstances.
- Make ``Future.add_done_callback`` work properly from inside
an asyncio event loop.
- Restrict cython to below 3.x (can't build with Cython 3 yet)

Added
- Fix Future's await implementation to be compatible with asyncio

Added
- chorde.client.asyncache.Future is now awaitable

Bugfixes
- Prevent memory leaks by clearing non-running traceback
frames from exceptions set as future results.

Bugfixes
- Allow sPickle unpicklers to accept strings as checksum keys